1. Field of the Invention
This invention relates to a cellular communication network and its communication method, and more particularly an overlay type cellular communication network, which has a configuration in which cells having different cell sizes overlap, and its communication method. Further, this invention relates to a cellular communication network using both free space light and a radio wave as a transmission medium. Further this invention relates to a wide area communication network and a local area network (LAN). This invention also relates to a routing technology to switch transmission routes of a packet, which is responsible for information transmission.
2. Description of the Related Art
Conventionally, a cellular communication network shown in FIG. 16 is widely known. Base stations 101a through 101d provide services in cells 103a through 103d, respectively. The base stations 101a through 101d are connected each other by a wiring network or a fixed wireless network (i.e. a micro wave network having directivity) 104. The base station 101a and a mobile station 102a communicate each other by means of a radio wave. Adjacent cells uses radio waves of different frequencies for communication between the base station and the mobile station, to avoid an interference between cells. A mobile station 102b which is near a boundary between cells, negotiates with the base stations 101a and 101b in order to select a base station to be linked from the base stations 101a and 101b. This communication process is called "hand-over". This communication network is widely used for a mobile telephone or a personal handy phone system (PHS).
As an extension of the above-described cellular communication network, a cellular communication network which integrates a macro cell indicating a big size cell and a micro cell indicating a small size cell, is proposed. For instance, Japanese Laid-Open Hei. 4-506294 proposes a system shown in FIG. 16, in which a communication satellite is employed as a macro cell 202 and base stations 101a through 101d built on the earth are employed as micro cells. In this system, a mobile station 102 is linked via a radio wave 204 to one of the base stations 101a through 101d in the area having high population, and is linked via a radio wave 203 to the communication satellite 201 in an area where it is unprofitable to build a base station because of its small population.
In this specification, the structure in which the micro cell is overlaid with the macro cell is called an overlay cellular communication network. Actually, this kind of overlay cellular communication network is spontaneously generated. For instance, when a PHS network whose cell size is a few hundred meters is built in an area where a mobile telephone network whose cell size is a few kilometers has been built, an overlay cellular communication network seems to be generated. These two networks are separately managed, however, they are not enough for a well-organized overlay cellular communication network. That is, a interconnection between the macro cell and the micro cell is not considered.
A system in which base stations are wired by optical fibers and optical signals directly modulated by radio waves are distributed among the base stations, is proposed as the above-described cellular communication network (Japanese Patent Laid-Open No. Hei. 6-311083, and Arai et. al., "Dynamic rage on Fiber-Radio Microcell Radio System", Proceeding of the 1994 IEICE Spring Conference B488 (1994)). To form the cellular communication network, a system which uses free space light for communication between the base station and the mobile station, is also proposed (Japanese Patent Laid-Open No. Hei. 3-91329). A intercommunicating system rather than a cellular communication network, which distributes an optical signal directly modulated by a radio wave via an optical fiber network is also proposed (Japanese Patent Laid-Open No. Hei. 6-164498).
Meanwhile, a system which integrates light and a radio wave has been proposed for use in a remote control system (Japanese Patent Laid-Open No. Hei. 2-162846). FIG. 17 shows a configuration of the system for remote control of a machine in an invisible area. A control signal from a remote controller 111 is transmitted to a machine 114 to be controlled, via interconnection devices 112 and 113. The control signal from the remote controller 111 is at first transmitted to an optical interface 115 of the interconnection device 112 by means of a free space light beam. The interconnection device 112 converts the free space light beam signal 123 into a radio wave 124, and transmits the produced signal to the interconnection device 113. The interconnection device 113 converts the radio wave signal 124 into a free space light beam signal 125, and transmits the produced signal to the machine 114. In Fig. 17, reference numerals 116 and 117 are antennas. Reference numeral 119 is an optical interface of the machine 114. A reference numeral 121 is a room where the remote controller 111 is. A reference numeral 122 is a room where the machine 114 is. A reference numeral 120 is a wall which separates the rooms 121 and 122.
As another example for use in a remote control system, Japanese Patent Laid-Open No. Hei. 2-235447 discloses a system which uses a radio wave to transmit a signal from a mobile station to a base station (interconnection device), and uses a free space light beam to transmit a signal from the base station (interconnection device) to the mobile station.
FIG. 18 shows a network (internetwork) having a configuration which connects two networks 131 and 132 by way of an interconnection device 130. Types of the interconnection device 130 are classified into a gateway, a router, a bridge and a repeater in accordance with OSI (Open Systems Interconnection) reference model. FIG. 19 shows a diagram which associates the gateway, the router, the bridge and the repeater, to the layers of OSI reference model. It is known that the gateway, the router, the bridge and the repeater have interconnection features corresponding to the application layer, the network layer, the data link layer (in particular, MAC: the media access control layer) and the physical layer of OSI reference model, respectively. Conventionally, interconnection devices (interconnection device 130), such as a gateway, a router, a bridge or a repeater, were connected to two networks simultaneously. Meanwhile, a general terminal (133 in FIG. 18) was not connected to two networks simultaneously.
FIG. 20 schematically shows how a packet is encapsulated and decomposed. At the transmission side, a packet is transmitted from the top layer to the bottom layer, successively, of the OSI reference model, and in each layer, a header of the layer (a session header, a transport header, a network header or a data link header) is added to the packet. At the reception side, a packet is transmitted from the bottom layer to the top layer of the OSI reference model, and in each layer, a header corresponding to the layer is removed from the packet. A typical gateway relays a packet after decomposing the packet into data entities. A router relays a packet after decomposing the received packet into a packet having a network header. A bridge and a repeater relay a packet without decomposing the packet itself. The repeater relays a packet without reference to contents of the data link header, while the bridge has a feature of packet filtering, which determines whether the packet is relayed or not in accordance with a MAC (media access control) address written in the data link header. A form of the packet (which header is attached to the packet) can be used to determine which layer of OSI reference model corresponds to a process.
FIG. 20 does not show a trailer corresponding to a header, which is sometimes attached to the end of the packet. That is, a trailer (session trailer, transport trailer, network trailer or data link trailer), which corresponds to a header (session header, transport header, network header and data link header), can be attached to the packet. A data entity is sandwiched between a header and trailer to be capsulated. Since the header is indispensable, but the trailer is sometimes unnecessary, FIG. 20 does not include trailers.
Since light is an electromagnetic wave having extremely high frequency, light can carry a broadband modulated signal. But light has a disadvantage that it is easily blocked by objects. Therefore, a cellular network using free space light has a disadvantage that a cell is small. In other words, more cells are required to cover a particular area.
In the remote control system, the above-described Japanese Patent Laid-Open Hei. 2-162846 proposes a system which uses both a radio wave and light, and compensates by a radio wave for a limit of distance which free space light can reach. Since this system simply replaces a particular signal route with a radio wave, however, it is insufficient to cover the whole of a relatively wide area as a cellular network.
In a general cellular network, the above-described communication process, what is called hand-over, is a control process carried out between plural base stations and a mobile station. Accordingly, there are problems that traffic between base stations increases and each base station must have an ability of information processing such as address management.